S7E battery suddenly started draining fast?

A year long S7E user here. In that year, I have been charging it every day wirelessly to around 90% and almost never let it go below 20%.

From last week, the phone started draining the battery quite fast. In some cases it goes down 1% every 40-50 seconds while I am browsing the internet. I am mostly using wi-fi in my bus to uni and more often 4G. 4G, combined with network coverage (i travel between cities daily for around 3 hours) seems to drain it the most. That's why when I am on wi-fi, i usually put it on Airplane mode so it doesn't drain it from network coverage.

However, when I go to battery usage, it says Google Play Services take up to 30% of the usage, followed by Screen (23%), Device Idle (15%) and Cell Standby (15%).

I have disabled many notifications, AVG's shields, scans (etc), regularly do device maintenance, even did a factory reset and the problem still stays.

On a daily basis, the battery used to drop from 90% to around 40% with heavy usage. Now it drops to 20%, sometimes even below that.